This invention relates to color video projection display systems and more particularly to optical pathway components for use in projection systems employing reflective liquid crystal on silicon (xe2x80x9cLCOSxe2x80x9d) light valves.
Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, and classroom training. In typical operation, multimedia projection systems receive analog video signals from a personal computer and convert the video signals to digital information to control one or more digitally driven light valves. Depending on the cost, brightness, and image quality goals of the particular projector, the light valves may be of various sizes and resolutions, be transmissive or reflective, and be employed in single or multiple light path configurations.
Recently, more optimal sets of multimedia projector characteristics have been achieved by employing reflective LCOS light valves. There are five general architectures for employing reflective LCOS light valves. The first employs a polarization beam splitter (xe2x80x9cPBSxe2x80x9d) cube prism and a so-called Philips prism; the second employs a PBS cube prism, a dichroic prism, and spectrally selective wave plates; the third employs multiple PBS cube prisms; the fourth employs a PBS cube prism and tilted plates; and the fifth employs an off-axis design implemented with linear polarizers, as opposed to PBS cube prisms. For each architecture, a number of variations exist, such as using crossed plates for color separation versus a solid xe2x80x9cX-cubexe2x80x9d prism color separator, using liquid filled PBS cubes instead of glass PBS cubes, and using additional polarizers or wave plates. However, each of the five architectures is generally distinct from the others and from the invention described herein.
All of the above architectures employ linear polarized light-sensitive devices for receiving light from a light source, reflecting the light off the LCOS light valves, and redirecting the reflected light, depending on its polarization direction, either out through a projection lens or back toward the light source. The polarization direction of the light is determined by an electronic image pattern applied to the LCOS light valve. To achieve a dark polarization direction, selected LCOS light valve pixels do not change the reflected light polarization direction, so the light returns to the light source and does not project toward the screen. To achieve an illuminated polarization direction, selected LCOS light valve pixels rotate the polarization direction by 90xc2x0, so the light is directed through the projection lens toward the screen. Projected image quality largely depends on how well the various optical path components establish, maintain, and analyze the light polarization directions. Image brightness largely depends on minimizing light loss through the various optical path components and polarizers.
In particular, the architecture employing a PBS cube prism and a Philips prism is described in U.S. Pat. No. 5,777,789 for EFFICIENT OPTICAL SYSTEM FOR A HIGH RESOLUTION PROJECTION DISPLAY EMPLOYING REFLECTION LIGHT VALVES, in which a cube PBS allows only linearly polarized light to propagate to a color splitting/combining prism. After reflecting from the light valves, the light is xe2x80x9canalyzedxe2x80x9d by the PBS cube and redirected according to the polarization direction of the analyzed light. This architecture is disadvantageous because it requires sophisticated optical coatings and non-standard prism angles and has skew ray depolarization caused by the PBS cube prism, stress birefringence caused by long path lengths in glass elements, and considerable weight due to the bulky prisms.
In the architecture employing a PBS cube prism, a dichroic prism, and spectrally selective wave plates, linearly polarized light is first incident on a spectrally selective half-wave plate that changes the polarization direction by 90xc2x0 for one color band only. A PBS cube separates the rotated color band from the un-rotated color bands based on their orthogonal polarization directions. Typically the green band is selected as the rotated color band because a dichroic cube splitter relatively easily separates the widely spaced wavelengths of the blue and red bands. After reflection from the light valves, the PBS cube analyzes the light, directs it according to its polarization direction, and recombines the color bands. Because the PBS cube has a non-ideal spectral response, a spectrally selective half-wave plate is required at the output face of the PBS cube so that all three color bands have the same polarization direction after passing through the wave plate and can, therefore, all pass through a xe2x80x9cclean-upxe2x80x9d polarizer. This architecture is disadvantageous because of stress birefringence caused by the large path lengths in glass, skew ray depolarization caused by the PBS cube prism, and considerable weight due to the bulky prisms.
In the architecture employing multiple PBS cube prisms, light is separated into red, green, and blue light paths using dichroic filter plates. Each of the three color paths contains a PBS cube, and each PBS cube allows only linearly polarized light to pass through to an associated light valve. Light reflected from the light valves is xe2x80x9canalyzedxe2x80x9d by the respective PBS cube and redirected according to the polarization direction of the analyzed light. For each color path, light propagating toward the projection lens is recombined with light from the other color paths via an X-cube prism. This architecture is disadvantageous because of considerable aggregate weight of the three PBS cube prisms and the X-cube prism, high component cost and complexity, stress birefringence, skew ray depolarization in the PBS cube prisms, and a large footprint created by the separated color paths.
In the architecture employing a PBS cube prism and tilted plates, the PBS cube prism allows only linearly polarized light to propagate toward a set of tilted dichroic filter plates. The first plate reflects one color band and passes the remaining light to the second dichroic filter plate, where it is further split into two more color bands. After reflection from the light valves, the color bands of light retrace their paths and recombine via the color splitting plates. The light is subsequently xe2x80x9canalyzedxe2x80x9d by the PBS cube, and redirected according to the polarization direction of the analyzed light. This architecture is disadvantageous because the PBS cube prism is bulky, heavy, has stress birefringence, and skew ray depolarization, and the projection lens requires a long back working distance.
The architecture employing an off-axis design and linear polarizers is described in xe2x80x9cProjection Displays V,xe2x80x9d SPIE Proceedings, January 1999, Vol. 3634, pp. 80-86. This architecture employs a two-level arrangement in which the incoming light propagates upwardly at an angle and through crossed dichroic color splitting plates. A sheet type linear polarizer positioned in each color path polarizes the light. The polarized light continues to propagate upwardly and reflects off the light valves. The polarization direction of the light is analyzed by another sheet type linear polarizer in each color path. Light reflected by dark polarization direction pixels undergoes absorption in the polarizer, and light reflected by illuminated polarization direction pixels propagates through the polarizer to an X-cube prism color combiner. This architecture is disadvantageous because it has an unduly high-profile, two-level form factor and requires an proprietary, asymmetrical, off-axis projection lens.
What is needed, therefore, is a compact, low-profile multimedia projection system that achieves a bright, high-quality projected image at a relatively low cost.
An object of this invention is, therefore, to provide a reflective LCD light valve-based multimedia projector.
Another object of this invention is to provide a multimedia projector that is lighter weight, more compact, potentially less costly, and of simpler optical design than prior prism based projectors.
A further object of this invention is to provide a multimedia projector having a three-path reflective light valve assembly that requires no prisms to form multi-color images.
Still another object of this invention is to provide a multimedia projector that has a simplified optical system employing a plate-type transflective polarizing beam splitter, spectrally selective waveplates, and a dichroic filter.
The following descriptions of preferred embodiments of this invention refer to P-polarized light and S-polarized light. P-polarized light has a polarization pass orientation in the plane of incidence and reflection, and S-polarized light has a polarization pass orientation that is parallel to the surface of an optical element, i.e., is orthogonal to the plane of incidence and reflection.
A first preferred embodiment of an image projector of this invention includes a light source that illuminates a three-path reflective LCD assembly that produces images for projection by a projection lens. The light source provides S-polarized light rays that are received by a spectrally selective input wave plate that changes a first wavelength range of light rays to P-polarized light rays and propagates without polarization change second and third wavelength ranges of light rays. A plate-type transflective polarizing beam splitter transmits the P-polarized first wavelength range light rays and reflects the S-polarized second and third wavelength range light rays. The P-polarized first wavelength range light rays propagate through a field lens and impinge on a first reflective LCD light valve. The S-polarized second and third wavelength range light rays strike a pleochroic, and preferably dichroic, filter, which splits them into second and third wavelength range light rays that propagate through field lenses and impinge on respective second and third reflective LCD light valves.
The P-polarized first wavelength range light rays impinging on dark state pixels of the first LCD light valve are reflected without changing polarization direction and return along their original paths through the transflective polarizing beam splitter toward the light source. The S-polarized second and third wavelength range light rays impinging on dark state pixels of the respective second and third LCD light valves are reflected without changing polarization direction, are recombined by the dichroic filter, and return toward the light source by reflecting off the transflective polarizing beam splitter.
The P-polarized first wavelength range light rays impinging on illuminated state pixels of the first LCD light valve are reflected with a 90xc2x0 change in polarization direction and are reflected toward the projection lens by the transflective polarizing beam splitter. The S-polarized second and third wavelength range light rays impinging on illuminated state pixels of the respective second and third LCD light valves are reflected with a 90xc2x0 change in polarization direction, are recombined by the dichroic filter, and transmit through the transflective polarizing beam splitter toward the projection lens.
The first, second, and third wavelength ranges of light subsequently propagate to a spectrally selective output wave plate that changes the S-polarized first wavelength range of light to P-polarized light, but does not change the polarization states of the second and third wavelength ranges of light. After propagating through the spectrally selective output wave plate, all three wavelength ranges of light have substantially the same polarization direction. A xe2x80x9cclean-upxe2x80x9d linear polarizer oriented with its transmission axis parallel to the first, second, and third wavelengths of light blocks any light having an undesired polarization direction resulting primarily from non-ideal light transmission and reflection characteristics of the transflective polarizing beam splitter.
A second preferred embodiment of an image projector of this invention includes an optical system that is constructed similarly to the first embodiment but employs randomly polarized light from the light source and does not require the spectrally selective input wave plate. Accordingly, the randomly polarized light rays propagate toward the transflective polarizing beam splitter, which propagates P-polarized ones of the light rays toward a first field lens, and reflects S-polarized ones of the light rays toward a dichroic filter. The dichroic filter transmits S-polarized first and second wavelength range light rays toward the second field lens and reflects S-polarized third wavelength range light rays toward the third field lens.
The light paths associated with the field lenses each include an optional dichroic trim filter placed in the path of the associated LCD light valves. The trim filters reflect selected wavelength ranges of light rays to perform a color balancing function without changing their polarization directions, so the reflected light simply returns toward the light source. Light rays having the desired wavelength range transmit through the trim filters for reflection by pixels of the LCDs light valves. In this way each of the three LCD light valves receives and reflects its respective one of the first, second, and third wavelength ranges of light rays. The remainder of the second embodiment is constructed similarly to the first embodiment.
This invention is advantageous because it enables constructing a reflective LCD light valve-based multimedia projector that is lighter weight, more compact, potentially less costly, and of simpler optical design than prior prism based projectors. The system is lighter because no bulky prisms are required, more compact because it requires only one dichroic filter and a polarizing beam splitter, and less costly because the optical component count is low and there are no prisms.
This invention is further advantageous because it allows construction of a projector that is less subject to contrast degradation due to birefringence problems caused by residual or thermally induced stresses.